Nitrogen in tires, instead of air.

[Northwoods]

All this talk of nitrogen fill vs. compressed air fill; how much the internal pressure of the filled tires varies with the change in ambiant temperature; final cost analysis of driving to a specialized N2 fill station vs. the corner gas station compressed air for a top off is all well and truely fascinating.

What I really want to know is how the air (or pure nitrogen) fill in the tire moves over time as the tire travels down the road.

Air has mass, so intertia must come into play as the tire starts to rotate in travel.  I can see the fill right next to the tire and the wheel being dragged along for the ride (so to speak) with the tire and wheel and and the wheel, but what about the air further away from the rotating surfaces?  How long does it take for that fill to come to a rotational equilibrium with the rest of the rotating body, and how long does it take to come to rest when the tire stops moving?

What about when the vehicle changes its rate of movement in stop and go events?  Does the density of the fill change over a unit area as the tire comes to a stop or starts from a stop?

While that might be interesting from "too much time on his hands academic" point of view, it would have no real-life appreciable effect.  The mass of air in the tire, assuming 70F, 30psig (45psia), an 18" interior OD, 15" interior ID, and 8" width is only 0.08lbs.  

[Tallpine]

I'm just amazed that back in the dark ages of the 20th century we managed to get along with just plain old compressed air in our tires. 

I can even remember using organic air from a hand pump 

[Perd Hapley]

Tallpine, you had it easy. We had to make our own.

[birdman]

While that might be interesting from "too much time on his hands academic" point of view, it would have no real-life appreciable effect.  The mass of air in the tire, assuming 70F, 30psig (45psia), an 18" interior OD, 15" interior ID, and 8" width is only 0.08lbs.  

That's a small low profile tire...it would be a 205/18-15!  A more typical (but still low profile) 205/50-15 would have about 1/3rd of a pound--still inconsequential, but more reasonable

[Tallpine]

Tallpine, you had it easy. We had to make our own.

That really blows 

[Perd Hapley]

Just be careful with the N2. Tweakers like to suck it out of your tires to get high.

[seeker_two]

I imagine that whole explosive/flammable thing might be a reason.

What's the worst that could happen?.....

[CNYCacher]

That's a small low profile tire...it would be a 205/18-15!  A more typical (but still low profile) 205/50-15 would have about 1/3rd of a pound--still inconsequential, but more reasonable

It's unsprung weight, and it's in the wheel, but at the same time it's not exactly rotational weight either because the air isn't going to rotationally accelerate right along with the wheel and tire, it'll lag behind.  Like when you rotate a drinking glass and the ice and water inside don't really move.

But, for sake of argument, lets assume the air mass is actually locked to the wheel and rubbber:

A fun math exercise would be to take the 1/3 lb, treat it as a evenly-distributed torroidal mass that its inside the center of the airspace of the tire, and measure how much power would be required to accelerate it up to whatever wheel RPM you have at the end of your best 1/4 mile run, in a time window equivalent to your ET for that run. Go ahead and add in the linear acceleration along the track for a true worst-case scenario.

If you figure that out, I would love to know what X is in the statement "During my best 1/4 mile run in the Skyline, X horsepower was used just to accelerate the air in the tires."

[Tallpine]

Math is still looking for his Ex 

[birdman]

It's unsprung weight, and it's in the wheel, but at the same time it's not exactly rotational weight either because the air isn't going to rotationally accelerate right along with the wheel and tire, it'll lag behind.  Like when you rotate a drinking glass and the ice and water inside don't really move.

But, for sake of argument, lets assume the air mass is actually locked to the wheel and rubbber:

A fun math exercise would be to take the 1/3 lb, treat it as a evenly-distributed torroidal mass that its inside the center of the airspace of the tire, and measure how much power would be required to accelerate it up to whatever wheel RPM you have at the end of your best 1/4 mile run, in a time window equivalent to your ET for that run. Go ahead and add in the linear acceleration along the track for a true worst-case scenario.

If you figure that out, I would love to know what X is in the statement "During my best 1/4 mile run in the Skyline, X horsepower was used just to accelerate the air in the tires."

Well...given the tires average to a 275/30-20 at 28psi, it's roughly 220g per tire so two pounds for all four...car with me in it is 4000.  For tire air (which will be at rotating speed with effectively no lag to answer that question) effective inertia (rotating plus static) is twice static, so 4 lbs effective, so it took 1/1000th of the power just to accelerate the air, or about 0.5HP. 
To put in perspective, that's about the same as the air resistance of a cyclist at 40, just to accelerate the air!

[Boomhauer]

The whole nitrogen scam reminds me of the time several years ago a tractor dealer down here sold the state a fleet of tractors that were a brand that used property grease fittings (which would have taken a $6 grease gun adapter to service). The dealer was able to convince the state people that the tractors HAD to be greased at the dealership and was able to charge big bucks to go get the tractors with a lowboy, take them back to the shop, and grease them and return.

[birdman]

Just to clarify:
Nitrogen for a normal car = scam
Nitrogen for race car = not scam

[rcnixon]

It's a load of crap and a way to separate fools from their money.

Russ

[birdman]

Again, for normal cars, yes. 

But to use the F1 argument,
Since there is no rule or argument mandating a pure N2 fill for tires, they wouldn't use it unless there was a performance advantage. 

Since you are unwilling to accept all the reasons so far, do you have a reason other than a scam, why, when performance at any cost is the goal, nitrogen is chosen for race cars?

[Boomhauer]

I don't drive a race car as my daily driver. Yes, there are performance advantages when you are talking speeds well in excess of our interstate speeds. I also don't wear a helmet or a 5 pt harness in my car, nor do I have a roll cage. Apples to oranges.

For our daily drivers, it's a scam, pure and simple.